1. Field of the Invention
The present invention relates to a sheet processing apparatus and an image forming apparatus having the same, particularly to a sheet processing apparatus for aligning sheets and binding them without causing misalignment of the rear ends.
2. Description of the Related Art
Conventionally, some image forming apparatuses such as copying machines are provided with a sheet processing apparatus which sequentially takes image-formed sheets into the apparatus and binds them in order to, for example, reduce the time required in binding the imaged-formed sheets.
For example, as disclosed in Japanese Patent Application Laid-Open No. 2004-59314, some of the sheet processing apparatuses carry the sheets by an intermediate roller to an intermediate sheet stack portion on which the sheet ends are aligned, and thereafter bind them using a binding unit such as a stapler.
FIG. 21 shows the configuration of this conventional sheet processing apparatus, in which an image-formed sheet S is conveyed to an intermediate sheet stack portion 101 by an intermediate roller 102. Then, the sheet S stacked on the intermediate sheet stack portion 101 is returned in the opposite direction to the conveyance direction by a not-illustrated return unit and contacts with an alignment reference wall 104 and the rear end in a conveyance direction is aligned.
However, if the precedent sheet S stacked on the intermediate sheet stack portion 101 is lifted above the nip line of the intermediate roller 102 when a sheet is conveyed, a subsequent sheet may collide with the precedent sheet S and disturb the alignment or cause a jam.
Therefore, conventionally, a rear-end regulation member 103 is used to prevent the rear end of the precedent sheet S stacked on the intermediate sheet stack portion 101 from moving (being lifted) by the rear-end regulation member 103. The rear-end regulation member 103 is rotatably set to the downstream of the nip position of the intermediate roller 102 and is energized in the direction opposite to the sheet conveyance direction shown by the arrow B so as to limit the position of the rear end of the precedent sheet S by the lower end and prevent the lift of the precedent sheet S.
Then, the rear end of the precedent sheet S is located below the nip line of the intermediate roller 102 before the rear-end regulation member 103 is pressed by the front end of a subsequent sheet to rotate. Thereby, when the rear-end regulation member 103 rotates, the front end of the subsequent sheet is conveyed up to the downstream side of the rear end of the precedent sheet S and the front end of the subsequent sheet does not collide with the rear end of the precedent sheet S.
Moreover, a conventional sheet processing apparatus is provided with a pressing unit 170 which presses the top of a precedent sheet S1 aligned in its end after the sheet is conveyed to the intermediate sheet stack portion 101 as shown in FIG. 22. Then, by pressing the precedent sheet S1 by the pressing unit 170, extrusion of the precedent sheet S1 by a subsequent sheet S2 is prevented.
In this case, the pressing unit 170 is lowered by the action of a solenoid 173. Then, when the pressing unit 170 moves downward, an elastic body 172 on the lower portion presses the top of the precedent sheet S1 to prevent extrusion of the precedent sheet S1 by a subsequent sheet S2.
When aligning the end of the sheet S1 conveyed to the intermediate sheet stack portion 101, the pressing unit 170 can rise to a retracting position by the action of the solenoid 173. Therefore, the pressing unit 170 does not interrupt alignment.
However, in the case of the conventional sheet processing apparatus and image forming apparatus, when curl is large, for example, at the traveling-directional upstream side of a sheet in the conveyance direction, the rear end of the precedent sheet S1 may be stacked so as to lean to the alignment reference wall 104.
In this case, because the rear-end regulation member 103 is not energized in the direction in which a sheet is pressed against the intermediate sheet stack portion 101, the precedent sheet S1 enters the gap between the alignment reference wall 104 and the rear-end regulation member 103. Then, when the precedent sheet S1 enters the gap between the alignment reference wall 104 and the rear-end regulation member 103, the rear-end regulation member 103 cannot regulate the rear end of the precedent sheet S1. As a result, a subsequent sheet collides with the rear end of the precedent sheet S1 and there is a problem that the sheet is damaged or jam occurs.
Moreover, in the case of the alignment reference wall 104, a portion 104a close to stapler frontage is lowered so as to lure a sheet conveyed by the intermediate roller 102 into the frontage of the stapler 142 located at a position lower than the nip of the intermediate roller 102. Therefore, this problem is significant particularly at a position close to the stapler frontage.
Therefore, to solve the above problem, a pressing member 171 for preventing lift of the rear end of a sheet may be set to the pressing unit 170 as shown in, for example, FIG. 25. Moreover, when the pressing unit 170 moves downward, the rear end of the precedent sheet S1 is pressed to the intermediate sheet stack portion 101 by the pressing member 171.
However, when the underside 171a of the rear-end regulation member 171 completely presses the rear end of the precedent sheet S1 in order to press the rear end of the precedent sheet S1 to the intermediate sheet stack portion 101, the sheet rear end is buckled before pressing the curl of a sheet as shown in FIG. 25. Thereby, the rear end of the sheet leans to the first alignment reference wall 104. When bringing the sheet to a binding unit, misalignment of the rear end of a sheet bundle occurs.